It is the ultimate aim of this study to gain a better understanding of what constitutes the active site of insulin. In this study we will be using the methods of peptide chemistry; i.e. semisynthesis and total synthesis in respect of the synthesis of each individual disulfide link. Our approach differs significantly from the previous attempted identification of active site residues in that we are trying to test whole areas of the insulin molecule and transplant those into a carrier of identical structure but different amino acid sequence. Such a carrier has fortuitously became available with the discovery that the hormone relaxin has a structure identical to that of insulin, but different amino acid sequence and no biological or immunological crossreactivity with insulin. Physical chemical and biological properties of the resulting hybrids will be determined. Should these study be successful a long-term object might be the design and synthesis of minimum insulin consisting of the active site and kept in the proper position by a minimum frame. Such a molecule might be relatively stable to influences such as acid, peptic and tryptic digests, and stable to the attack by exopeptidases. Furthermore, we hope that such a molecule might be designed to be absorbed readily through the gut and thus ultimatively provide a relief from daily injections for insulin- dependent diabetics.